Avalanche forecast

Banner Summit

Sawtooth Avalanche Center
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Avalanche danger

Wed
Alpine
3 · Considerable
Treeline
3 · Considerable
Below
2 · Moderate
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Out of date — this forecast expired 3 years ago. You’re viewing past conditions, not the current bulletin.

Highlights

You can trigger 2-3 foot thick slab avalanches that fail on widespread layers of weak snow. The likelihood of triggering slides will increase throughout the day with more snow and wind through the storm. Avalanches can be triggered from long distances away from steeper slopes. Smaller loose snow and wind slab avalanches involving the recent snow are also possible. These smaller natural and triggered slides can step down to the weak layers beneath, creating much larger, more destructive avalanches.

Avalanche problems

  • Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    2–3

    Large, destructive avalanches failing on layers of weak snow buried 2-3 feet beneath the snow surface continue to be a worrisome possibility here. Avalanches that failed on these layers this season have occurred across a range of aspects and elevations. Dig down and you'll find facets and/or surface hoar on shadier aspects and crust + facet combinations on sunnier slopes. These weak layers are impressively widespread and exist on most slopes.

    Slides that fail on these weak layers will break widely and can be triggered from long distances away from the slope that avalanches. Thanks to the very dense slab of snow on top of these weak layers, you aren't likely to get obvious signs of instability as you move through the mountains.  The first indication of instability may be the large, destructive avalanche you trigger. Travel through the mountains knowing that triggering these slides is on the menu.

    (1/22/2024) The debris of this slide ran into a creek, a lower and mid-elevation terrain trap that is characteristic of the Banner Summit Zone. Terrain features like these multiply the consequences of these slides.

  • Wind Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    1–1.5

    Overnight we saw sustained winds that brought with them up to 4 inches of snow. This fell in addition to the over a foot of light snow that was already available for transport at upper elevations from our last storm. Wind drifted this snow into sensitive slabs both at upper and exposed middle elevation terrain. Expect these pockets to be reactive to your body weight and larger as you climb higher in elevation. As wind and snowfall continue throughout the day, expect the size and reactivity of these slabs to increase. 

    Importantly, these smaller slides can trigger much larger, much more dangerous avalanches on the persistent weak layer described above. Keep an eye out for rippled surfaces, hollow-sounding stiff snow, and cracking shooting out from your skis. The low-density snowfall will mask the presence of these slabs. Avoid drifts, dunes, and pillows.

Avalanche Discussion

The mountains didn't need any more snow or wind to make things dangerous, but they're getting it anyway. This snow will build our slab to 2.5-4 feet thick in this zone, laying on top of an already stressed and impressively fragile widespread layer of weak snow. In three of the past four days, large to very large avalanches were remotely triggered near the boundary of the Galena Summit and Eastern Mtns and Soldier and Wood River Valley Mtns zones. Near Johnstone Peak three days ago, I was able to trigger a slide from nearly half a mile away that spanned multiple alpine bowls and moved the entire snowpack downhill. Slides like these will be more likely today, with the chances of triggering them increasing throughout the day as the snow falls and wind blows. The wind will build smaller but still consequential and reactive slabs at upper and exposed mid-elevations that are likely to step down to create larger avalanches if triggered.

While this zone is seeing less new snowfall during this storm, we still have a slab that is becoming increasingly hefty and dense on top of a widespread weak layer. The consequences of this are two-fold: and increased force and consequence of slides, and a decrease in warning signs to before triggering them. To put the power and scale of these slides into perspective,  the force of one of the dense slabs that failed about Trail Ck created when moving downhill triggered another large avalanche in unconnected terrain just from the impacts of the snow on the hillside. In areas where the snowpack has remained thinner (southern and eastern mountains) the slab is a bit thinner and triggering avalanches on it will be slightly easier. Across our region, any avalanche you trigger that involves this weak snow is likely to break hundreds of feet wide (or wider) and will send a majority of the snowpack rocketing downslope. Triggering one of these slides is likely today.

Yesterday was dangerous. Today is worse. Riding out up the drainage or starting up the skintrack, remind yourself that avalanches large enough to damage a house are likely. These slides can fail in places that you haven't seen them before and can pull back into lower angle terrain above steeper slopes. Avalanches can be triggered from large distances away, and can not only impact your group but also others around you. Heading into the mountains today, there are two ways your day in the backcountry could go: low angle powder skiing and riding, wiggling in safety with friends before coming home safely at the end of the day — or the alternative. 

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